Aircraft wheels are currently designed primarily in response to requirements for accommodating selected tires as well as housing an aircraft's brakes and supporting the aircraft on ground surfaces. A goal is to accomplish these functions with a wheel design that adds a minimum amount of weight to the aircraft while maximizing wheel operational life. Aircraft wheels not only help support the aircraft's weight during taxi, but, along with other landing gear structures, are required to absorb significant loads when an aircraft lands. The conditions under which aircraft wheels must operate during service, moreover, may be harsh and demanding. Aircraft wheels typically experience not only high energy braking events that can produce significant heat, but are also exposed to carbon dust from the application of carbon brakes and to fluids from runways and aircraft. Consequently, the United States Federal Aviation Administration (FAA) and corresponding international aircraft regulatory bodies require that each aircraft main and nose landing gear wheel be submitted for approval prior to use on an aircraft. The FAA reviews such factors as maximum static and limit load ratings of each wheel, taking into consideration design maximum weight and critical center of gravity. The maximum limit load rating, for example, must equal or exceed the maximum radial limit load determined under applicable ground load requirements of the wheel. Additional requirements apply to the approval of wheels with brakes.
Various wheel configurations are available that have been designed to meet these requirements. A wheel configuration known as an A-frame design, while structurally efficient and light in weight, provides limited space for brakes or other structures. Another type of wheel configuration known as a bowl-type design provides the space needed for aircraft brakes, but is heavier than the A-frame wheel. For large aircraft with high brake energy and heat sink requirements, however, a bowl-type wheel, although it may increase the aircraft's weight, may meet these needs.
The vast majority of aircraft wheels are made of forged aluminum alloys. Some steel and magnesium alloys previously used to make wheels are heavier than aluminum and may, in addition, present corrosion challenges. Titanium has been proposed for aircraft wheels, but has not been found to be a practical substitute for aluminum alloys. The cost of making aircraft wheels from titanium has been found to be significantly higher than the cost of making aircraft wheels from aluminum. One comparison of aircraft wheel assembly unit weight, however, found an aircraft wheel assembly made of titanium to have a low weight advantage over both forged and cast aluminum. Because aircraft wheels are among the most highly stressed parts of an aircraft and the complete failure of an aircraft wheel can be catastrophic, the material selected to form an aircraft wheel must be able to withstand any loads and stresses to which the wheel is likely to be subjected. One material that has been proposed for this purpose is Aluminum Alloy 2040, which has a relative composition of Al-5, Cu-0.8, Mg-0.6, Mn-0.5, Ag-0.122.
The prior art has proposed different approaches to aircraft wheel design and materials to reduce weight while maintaining the requisite strength to sustain loads encountered during operation. In U.S. Patent Application Publication No. US2010/0001130, for example, Steinke et al describe a hybrid aircraft wheel in which a wheel section with a rim flange is formed of an epoxy resin composite reinforced with carbon fibers and metal. This construction is stated to be lighter than aluminum and strong enough for aircraft applications, including a nose wheel or a non-braked main wheel. In U.S. Pat. No. 5,018,795, Engerand et al describe an aircraft wheel formed of both metal and a composite material made with high strength carbon fibers bonded by an organic resin designed with a removable beading mounted by a titanium mounting belt. This arrangement is stated to have sufficient strength to withstand stresses to which aircraft wheels are subjected as well as to permit a tire to be mounted easily on the wheel. Using combinations of metals with functional characteristics selected to correspond to specific wheel section functions is not suggested.
Providing motors and other drive devices to move aircraft independently on the ground without reliance on the aircraft's main engines or tow vehicles is known. Such motors and drive devices may be installed in aircraft wheels to drive or power the wheels and thereby move the aircraft during taxi. In U.S. Pat. No. 7,445,178, for example, McCoskey et al describe a powered nose aircraft wheel system with a traction motor mounted partially in an A-frame type wheel that rotates the wheel to drive the aircraft on the ground. Wilson, in U.S. Patent Application Publication No. US2011/0089289, describes mounting an electric motor in a two-part bowl-type aircraft main wheel in space normally occupied by brakes to drive the aircraft during taxi. An alternate brake location may be required with this design. Neither McCoskey et al nor Wilson, however, is concerned with optimizing wheel design or operation, and neither includes any reference to or description of materials used to make the disclosed drive wheel designs.
A need exists, therefore, for an aircraft wheel powered by non-engine drive means for autonomous aircraft ground movement in which design, construction, and functional properties of materials are selected to optimize operation of the wheel to support and move the aircraft on a ground surface.